harmonic design

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Harmonic Design Considerat i ons

Michael Leporace – Specification Engineer, GE Consumer & Industrial

ABSTRACT Power quality can be defined as the comparison of voltage and current waveforms to their fundamental

“clean” sinusoidal waveforms. Pure sine waves that are free of distortion are sometimes referred to as

clean power. Harmonic distortion is one of the main causes of power quality problems. Many papers

have been written on this subject, so only a brief review of the subject will be presented here. It is the

main intent of this paper to discuss some design ideas that should be considered when trying to decide

whether harmonic mitigation is necessary, and if so, what type should be implemented.

INTRODUCTION

A harmonic is a wave whose frequency is an integer multiple of the fundamental frequency. The French

mathematician Joseph Fourier coined the term ‘harmonics’ out of studies in relation to sound in the early

1800’s. From these studies, he developed methods to describe sets of complex repeating waveforms assums of simple sinusoidal waveforms. When the higher frequencies are added to the fundamental, it will

result in a distortion of the fundamental (see figure 1).

-1.5

-1

-0.5

0

0.5

1

1.5

0° 90° 180° 270° 360°

Degrees

A m p l i t u d e ( P . U . )

1st (60Hz)

3rd (180Hz)

5th (300Hz)

SUM 1st thru 5th

Figur e 1

1



Harmonics can be caused by magnetic saturation in the cores of transformers. They are also caused by

the switching action of electronic components such as diodes and thyristors, which are found in rectifiers

and Switch-Mode Power Supplies (SMPS). The primary component of a rectifier is a diode (see figure 2).

A diode is a device that allows current to pass through it in one direction when its terminal voltage (anode

with respect to cathode) is positive. It will continue to behave like a

closed switch and pass current until the current stops flowing. At thispoint it will behave like an open switch and block the flow of current. It

will only resume conduction when the anode again becomes positive

with respect to the cathode.

ANODE CATHODE

Figure 2

When diodes are connected as in figure 3, the output will be DC. This configuration is called a 6-pulse

bridge rectifier. There will be a ripple

unless filtering is applied. The bold

portions of the sine waves in figure 4

represent the unfiltered DC ripple. A

capacitor bank and some type of

filtering are usually fitted and the

theoretical DC output is equal to

approximately 1.35x the RMS (Root-

Mean-Square) supply voltage. This

diode configuration is commonly found

in AC Variable Frequency Drives

(VFD’s). But it is the switching action of

the diodes, which allow current to be

drawn from the supply in pulses that

causes disturbances to the supply. The

size and quantity of the loads connected

to the diode bridges in relation to the

size of the supply power generation

equipment, will determine the degree of

distortion. For example, a personal

computer draws a relatively small

amount of current through its SMPS.

So one computer will not cause a significant disturbance to the supply power. But a large office building

that contains hundreds of computers will cause a significant disturbance to the supply power unless

harmonic mitigating equipment is installed.

6-PULSE RECTIFIER

3-PHASE

AC SUPPLY

SUPPLY VOLTAGE WAVEFORM AT

INPUT OF RECTIFIER

DC LINK VOLTAGE

DC VOLTAGE

Figure 3

L1

L2

L3

It is not always practical and

sometimes not necessary to eliminate

harmonics. IEEE Std 519-1992(Recommended Practices and

Requirements for Harmonic Control in

Electrical Power Systems) lists

voltage and current distortion levels

that they recommend should not be

exceeded at the Point of Common

Coupling (PCC). (See Table 1.). The

LINE TO LINE

VOLTAGES

Figure 4

2



PCC is the point in the electrical system where ownership changes from the electric utility to the

customer. Since the utility company is expected to supply clean power to all of its customers, this point is

chosen in part so that each customer is responsible for minimizing their own harmonics to a level that will

not adversely affect other users. It is also intended to prevent the utility company from having to

purchase and install expensive harmonic mitigating equipment to correct any significant customer-

generated harmonics.

Table 1

Isc /IL <11 11≤h<17 <17≤h<23 <23≤h<35 35≤h TDD

<20* 4.0 2.0 1.5 0.6 0.3 5.0

20<50 7.0 3.5 2.5 1.0 0.5 8.0

50<100 10.0 4.5 4.0 1.5 0.7 12.0

100<1000 12.0 5.5 5.0 2.0 1.0 15.0>1000 15.0 7.0 6.0 2.5 1.4 20.0

Even harmonics are limited to 25% of the odd harmonic limits above.

Current distortions that result in a dc offset, e.g., half-wave converters, are not

allowed.

*All power generation equipment is limited to these values of current distortion,

regardless of actual Isc /IL.

where

Isc = maximum short circuit current at PCC.

IL = maximum demand load current (fundamental frequency component) at

PCC.

Table 10.3

Current Distortion Limits for General Distribution Systems

Maximum Harmonic Current Distortion

Individual Harmonic Order (Odd Harmonics)

120 V Through 69,000 V

in Percent of IL

Typically the utility’s power generation equipment will be capable of delivering much more power than a

facility’s generator that is primarily used to supply emergency power. If the Total Harmonic Distortion

(THD) is within acceptable limits while a facility is connected to the utility grid, then the facility need only to

concern itself with the effects of harmonics on its own equipment while connected to the utility’s grid. But

a design engineer responsible for the power quality of that facility will also need to be aware of the affects

that harmonics can produce when the facility is under generator power. For example, some phase

detection relays can misinterpret poor power quality caused by harmonics as an undervoltage or phase

failure condition.

3



CASE STUDY – SOFT STARTERS

Induction motors are the motors most widely used in industry. They are rugged, low-cost, and easy to

maintain. If an induction motor is started across the line, it will draw a high in-rush current (locked-rotor

current) that is typically 6 to 13 times its full load running amps. The load will also experience a high

torque pulsation at the time that the motor is first started. Locked–rotor current is proportional to the

voltage. Locked-rotor torque is

proportional to the square of the

voltage. Reduced Voltage Solid

State (RVSS) starters (a.k.a. soft

starters) contain thyristors

connected in an anti-parallel

configuration. An RVSS starter

varies the firing angle of the

thyristors to vary the voltage

supplied to the motor. By reducing

the voltage, they are able to reduce

the locked-rotor current and torque.Soft starters can also ramp a motor

down from full speed to zero speed.

But they are not used to continually

vary the speed of a motor.

During the ramping up or down,

significant harmonics can be generated. There are a few things that should be considered when soft

starters are installed:

MOTOR3-PHASE

AC SUPPLY

BYPASS

CONTACTOR

TWO ANTI-PARALLEL THYRISTERS

TYPICAL SOFT START CIRCUIT

Figure 5

ISOLATION

CONTACTOR

• How big is the soft starter/motor load in relation to the available supply?

• How often will the soft starter/motor load(s) be started?

• How long are the acceleration/deceleration ramps?

If the soft start/motor load is large and the motor will be started often with a long ramp time, then

harmonic mitigation equipment should be considered. But if the opposite is the case, and the facility’s

remaining equipment is robust, then the design engineer may choose not to specify that harmonic

mitigation equipment be installed.

A case in point was that of an airplane refueling facility. The facility had three 125hp refueling pump

motors, which employed soft starters to smoothly ramp the motors up to speed. Each motor circuit

contained a phase failure relay that was designed to prevent the motor from operating in the event of a

phase failure or undervoltage condition. When the utility company was supplying the facility’s power, the

voltage distortion was at a tolerable level and the phase failure relays did not trip. Figure 6 contains aphase to ground voltage waveform captured during the soft start ramp while under utility power. When

the refueling facility was being powered by their generator, the switching action of the soft starters

distorted the voltage enough to cause the phase failure relays to trip during the ramping. Figure 7 depicts

the voltage waveform when ramping under generator power. (Note: These measurements were taken at

the input to the soft starter, not at the PCC.).

4



Figure 6

Figur e 7

An examination of the two figures reveals that for relatively the same amount of current distortion caused

by the soft starter/motor (the red waveforms in both figures), the utility-supplied voltage (blue waveform)

was much less affected as compared to when the generator was supplying the power to the facility. Thefacility determined that their equipment was robust enough to handle the occasional excessive harmonic

distortion that occurs while the soft starter is ramping; so the solution was to bypass the phase failure

relays only while the soft starter was in operation under generator power.

5



EQUIPMENT SOLUTIONS

Passive Filters

A passive harmonic filter is typically an inductor connected in series with capacitors. It is also known as a

tuned filter because it will absorb whatever harmonic it is designed to filter out based on the values of the

inductor and capacitor(s). A passive filter is a relatively low-cost filter. But a disadvantage of this filter is

that it cannot absorb other harmonics that it is not designed to, and it cannot adapt to changes in the

electrical system. Therefore, if other harmonic frequencies need to be filtered, additional passive filters

tuned to absorb those frequencies will have to be added. Passive filters are a good, low-cost solution

when tuning for specific harmonic frequencies usually produced by a specific piece of equipment. If a

passive filter is determined to be the best solution for a facility, a harmonic study of the facility will need to

be performed. GE has engineers that can perform the study and design a filter to suit your needs. A GE

Auto GEMTrap Filter Bank is a passive harmonic filter that can also dynamically correct for power factor.

This is a great product for facilities with a significant amount of motor loads (corrected by the power factor

correcting capacitors) and fixed harmonic-generating loads (corrected by the filter).

citors. It is also known as a

tuned filter because it will absorb whatever harmonic it is designed to filter out based on the values of the

inductor and capacitor(s). A passive filter is a relatively low-cost filter. But a disadvantage of this filter is

that it cannot absorb other harmonics that it is not designed to, and it cannot adapt to changes in the

electrical system. Therefore, if other harmonic frequencies need to be filtered, additional passive filters

tuned to absorb those frequencies will have to be added. Passive filters are a good, low-cost solution

when tuning for specific harmonic frequencies usually produced by a specific piece of equipment. If a

passive filter is determined to be the best solution for a facility, a harmonic study of the facility will need to

be performed. GE has engineers that can perform the study and design a filter to suit your needs. A GE

Auto GEMTrap Filter Bank is a passive harmonic filter that can also dynamically correct for power factor.

This is a great product for facilities with a significant amount of motor loads (corrected by the power factor

correcting capacitors) and fixed harmonic-generating loads (corrected by the filter).

Multipulse TransformersMultipulse Transformers

Multipulse transformers (transformers providing more than six pulses of DC per cycle) are usually

employed in installations involving equipment fed from multiple 6-pulse rectifier bridges. Common

multipulse configurations are 12-pulse, 18-pulse, and 24-pulse. The greater number of pulses will result

in a reduction of the input line current distortion, which in turn reduces the voltage distortion. A multipulse

transformer is a phase-shifting transformer that has a 3-phase primary, and multiple secondaries. For

example, a 12-pulse transformer will have a delta primary that allows for connection to a 3-phase supply,

and a delta and wye secondary that allows for connection to two 6-pulse diode bridges that are connected

in parallel (see figure 8) or series.

Multipulse transformers (transformers providing more than six pulses of DC per cycle) are usually

employed in installations involving equipment fed from multiple 6-pulse rectifier bridges. Common

multipulse configurations are 12-pulse, 18-pulse, and 24-pulse. The greater number of pulses will result

in a reduction of the input line current distortion, which in turn reduces the voltage distortion. A multipulse

transformer is a phase-shifting transformer that has a 3-phase primary, and multiple secondaries. For

example, a 12-pulse transformer will have a delta primary that allows for connection to a 3-phase supply,

and a delta and wye secondary that allows for connection to two 6-pulse diode bridges that are connected

in parallel (see figure 8) or series.

Figure 8

The designer of the transformer will determine the amount of phase-shift needed based on the number of

6-pulse converters.

The designer of the transformer will determine the amount of phase-shift needed based on the number of

6-pulse converters.

6



A formula to determine which harmonics are significant (not cancelled by transformer/rectifier) is as

follows:

pulsesof numberp

numberintegerann

:where

1

=

=

±= npharmonict significan

Therefore, a 12-pulse system will still produce significant 11th, 13

th, 23

rd, 25

th, etc. harmonics. But it will

cancel the 5th, 7

th, 17

th, 19

th, etc.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Fundamental X

6-Pulse X X X X X X X X X

12-Pulse X X X X X

18-Pulse X X X

24-Pulse X X X

Signi f icant Harm onics f rom Br idge Rect i f ie rs

Table 2

Many multipulse configurations have been designed to be connected to VFD’s. It is a good solution for

mitigating harmonics on these known harmonic-producing products.

Harmonic Mitigation Transformers

Harmonic Mitigation Transformers (HMT) are transformers specifically designed to reduce system

voltage distortion and reduce zero sequence harmonic currents also known as triplens. Triplens are third

harmonics and all of their odd multiples (3rd

, 9th, 15

th, etc.). The mathematical phase properties of triplens

are such that they add together in the neutral wire instead of cancelling each other out. The design

nature of conventional, non-phase-shifting transformers and autotransformers (even phase-shifting

autotransformers) does not address the mitigation of triplens. HMT’s are designed to reduce triplens, and

3-PHASE LOAD;

0° PHASE SHIFT

FROM SUPPLY

3-PHASE LOAD;

30° PHASE SHIFT

FROM SUPPLY

Figure 9

7



8

when designed with other HMT’s as part of a system, they can reduce other harmonics. A system of

HMT’s would consist of transformers of different phase shifts. Together their effect would be similar to

that of a multipulse transformer—thus resulting in a large amount of harmonic cancellation. An example

would be to fit a delta/delta transformer (0° phase shift) on the same supply as a delta/wye transformer

(see figure 9). The result would be harmonic cancellation similar to that of a 12-pulse transformer,

(assuming relatively balanced loads).

Active Filters

Active filters such as GE’s GEMActive are a great solution for the reduction of harmonics, especially when

the offending harmonic frequencies are changing throughout the day. The GEMActive cancels harmonics

by dynamically injecting out-of-phase harmonic current. This reduces current distortion, which

consequently reduces voltage distortion. Current transformers provide the GEMActive filter with the

feedback required for closed loop control. It is able to constantly adjust for changing levels of harmonics.

It cannot be overloaded. If the network’s harmonics are greater than what the filter can correct for, the

filter will supply its maximum harmonic-canceling current. If further reduction is necessary, multiple units

can be connected in parallel to increase compensation.

CONCLUSION

Power quality has always been, and will continue to be a major concern, especially with the increased

use of sensitive electronic equipment. Due to their energy savings nature, harmonic-producing devices

will be installed more frequently. Since engineering and equipment costs will always be subjects that

facility owners and planners will try to minimize, understanding and measuring harmonics are very

important. Knowing what harmonic levels a system can safely tolerate can result in significant cost

savings. And knowing when harmonic mitigation equipment needs to be installed, as well as what type of

mitigation is necessary, can reduce equipment failures and expensive downtimes. GE can help owners

and planners understand and measure a facility’s harmonics, and can help design a cost-effective

solution. Consult your local GE sales office for more information.

REFERENCES

IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems,

IEEE Std 519-1992

Power Electronic Converter Harmonics, Derek A. Paice

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